Field of the Invention
The present invention relates to a subject tracking apparatus, a control method, an image processing apparatus, and an image pickup apparatus, and more specifically, a subject tracking apparatus for tracking a subject included in images which are sequentially supplied.
Description of the Related Art
In recent years, a technique in which a particular subject is extracted from images supplied sequentially in a time series manner and then, the extracted subject is tracked has been very useful. For example, the technique has been used for specifying a human face region and a human body region in dynamic images. Such a technique can be used in a number of different fields, such as teleconferences, man-machine interfaces, security, monitoring systems for tracking any subjects, and image compression.
Also, in digital still cameras, digital video cameras and the like, there is a well-known technique for extracting and tracking any subject included in a photographing image, and thereby optimizing a focal point state and an exposure state with respect to the subject (see, Japanese Patent Laid-Open No. 2005-318554, Japanese Patent Laid-Open No. 2001-060269, and Japanese Patent Laid-Open No. 2004-348273).
For example, Japanese Patent Laid-Open No. 2005-318554 discloses an image pickup apparatus for detecting (extracting) and tracking, with respect to the face, a position of a face in a photographed image and imaging the face at optical exposure while fitting the face to a focal point. Tracking the detected face enables stable control in a time series manner.
Also, Japanese Patent Laid-Open No. 2001-060269 discloses a technique such that a certain subject is automatically tracked by utilizing a template matching. The template matching is a technique for registering a partial image as a template image by clipping an image area with a certain subject set as a target to be tracked, and estimating an area with the highest degree of similarity, or the least degree of difference relative to the template image, and tracking the certain subject.
Japanese Patent Laid-Open No. 2004-348273 discloses, in contrast to the template matching, a technique, referred to as a histogram matching, for utilizing a histogram with respect to the amount of characteristic for the matching, rather than image data itself. This is a technique for converting information indicative of a subject from image data into a histogram, registering the histogram, estimating an area capable of being converted into the histogram most similar to the registered histogram among the images, and tracking the certain subject.
However, while template matching is good at classification between subjects that are similar to each other, it is weak at classification when there are changes in the appearance because template matching uses patterns of the image data as the amount of characteristic. In contrast, histogram matching can realize robust tracking of the subject with respect to changes in the appearance, such as a change in the attitude of the subject, by converting information indicative of the subject from the image data into a histogram for the sake of ambiguity. However, it is weak at the classification between the similar subjects. In addition, it is difficult to use histogram matching simply in combination with the template matching because the properties thereof are different from each other.